Years of production experience have shown that close reservoir supervision and relatively minor well intervention procedures may dramatically increase the amount of oil recovered from a given reservoir. As one example, for water drive reservoirs the standard method of production is to perforate the pay zone an optimal distance above the oil-water contact. As the oil-water contact moves up, the bottom perforations are squeezed off. If necessary, new perforations are added higher in the wellbore. This method allows more reserves to be recovered in a shorter period of time and reduces the costs associated with disposing of produced salt water. In a low-pressure reservoir, this method may prove critical to the economic viability of the well. Slight increases in the WOR may greatly reduce the production rate or even kill the well.
For wells located on land or in and shallow water, close reservoir supervision and minor well intervention work is common practice. In deep-water subsea wells, however, the cost of performing even a minor well intervention is very high. The standard practice for performing a minor intervention requires a drilling rig to be mobilized on the well and riser to be run. In 7,500 feet of water, just the time to deploy and retrieve the riser may be 4 to 5 days each way. At a day rate of $250,000 to $300,000, the cost of even a simple workover can be in the range of $6,000,000 or more. This high cost often makes simple intervention work prohibitively expensive. Also, there may be a considerable amount of production time lost while awaiting the availability of a suitable deepwater rig.
A major oil company recognized the need for an alternative technique for performing intervention work on subsea wells using coiled tubing technology and contracted Applicant to run a development project to create and commercialize such a device. The gated project consists of three major phases: feasibility, detailed design, and manufacturing and testing. The primary goal for the first phase of the subsea intervention module (SIM) project was to perform a sufficient amount of engineering and design work to verify the feasibility of the system.
The subsea intervention module (SIM) is a subsea coiled tubing unit envisioned to provide an economical means for servicing subsea wells. Initially, the SIM was to be assembled and deployed off the back of a large workboat. After the requirements for the SIM were more fully defined, the size and weight of the SIM practically may require the use of a ship with a large moonpool.
U.S. Pat. No. 4,054,104 discloses the submarine well drilling system with drill pipes restored in a submerged vessel.
U.S. Pat. No. 4,899,823 discloses a method of placing a coiled tubing or wireline reel and injector on the deployment vessel and blow out preventers (BOP's), strippers, and a second injector subsea. While this solution provides an incremental step change, it requires the injector and lubricator travel back to the deployment vessel every time a new tool is used.
In the ExxonMobil provisional Patent Application No. 60/224,720, all of the required equipment is located subsea. This patent application presents the concept of a tool caddy device located between the stripper and BOP stack, allowing tools to be switched out subsea. The caddy consists of two sets of tubes containing the tools and capable of acting as pressure vessels. While this system is good, it requires at least 35 feet between the top of the BOP stack and the injector. It also adds a significant amount of weight to the structure and is somewhat limited in the number of tools that it can carry. U.S. Pat. Nos. 6,488,093, 5,002,130, and 4,899,823, and publication PCT No. 01/00342, U.S. Ser. Nos. 97/17,219 and 99/11,811, as well as publication U.S. Ser. No. 2002/0,040,782 A1 disclosed various subsea intervention systems.
A conventional coiled tubing injector may be positioned at the surface of a land-based well or in relatively shallow water of an offshore well, although positioning of the tubing injector in a moderate or deep water well is impractical for most offshore coiled tubing operations. Some injectors have utilized sealed bearings for both land and shallow water operations. Conventional dynamic seals in sealed bearing packages cannot, however, reliably withstand the hydrostatic sea pressure and high operating speeds encountered for a coiled tubing injector working in a deep water environment. According to one proposal, the subsea tubing injector is protected from the subsea environment by an enclosure, with seals provided between the enclosure and the coiled tubing above and below the injector. An example of this system is discussed in U.S. Pat. No. 4,899,823.
Coiled tubing has been reliably used in land-based hydrocarbon recovery operations for decades, since various well treatment, stimulation, injection, and recovery operations may be more efficiently performed with conveyed coiled tubing than with threadably connected joints of tubulars. In a conventional land-based operation, the coiled tubing injector may utilize a gear drive mechanism with conventional bearing assemblies to reliably and efficiently transmit power to the coiled tubing.
Conventional pipeline practice involves the launching of pigs to perform maintenance operations on pipelines. A pigging loop provides a closed circuit for the pigs to be launched and retrieved. Pigging is typically done to remove debris, such as paraffin or sand, which restricts the flow of production. A significant drawback to conventional pipeline techniques is the additional capital cost of the pigging loop, and the likelihood pigs getting stuck in the pipeline.
The disadvantages of the prior art are overcomed by the present invention, and an improved subsea intervention system and method, and components of such a system, are disclosed below.